1. Field of the Invention
The present invention pertains to surgical knife blades and, more particularly, to a surgical knife blade for precisely forming incisions of predetermined lengths (sometimes referred to as incision widths) in bodily tissue and to a method of performing cataract surgery utilizing the surgical knife blade.
2. Discussion of the Prior Art
Many surgical procedures require the use of surgical knives for forming incisions in bodily tissue to provide access to a body cavity or internal operative site. Most surgical knives include a handle having a cutting blade mounted on a distal end thereof for being inserted in tissue to produce an incision. The cutting blades on surgical knives are usually of minimal thickness to penetrate tissue easily with minimal trauma such that the incisions can be closed without excessive stretching of surrounding tissue while promoting rapid healing with minimal scarring. The cutting blades on surgical knives are typically available in a variety of sizes and have a maximum width between opposing lateral sides of the blade, the blades being tapered to extend longitudinally in a distal direction from the area of maximum width to a sharp tip, or point, facilitating insertion of the blade in tissue. Accordingly, the lateral sides of the blades are configured to extend distally with a desired taper, and the exact configuration for the taper varies dependent upon the type of incision to be produced for a specific operative procedure or the desires of individual surgeons. When forming relatively long incisions, a blade is inserted in the bodily tissue in the manner of a plunge cut; and, once the tip or point has penetrated the tissue to the required depth for the operative procedure being performed, the blade is moved through the tissue in line with the lateral sides until an incision is produced having a length sufficient to provide access for the operative procedure. The length of the incision can be several times greater than the width of the blade; and, frequently, deviance of actual incision length from an optimal incision length for the operative procedure is not medically significant. However, many operative procedures require that the lengths of incisions be precisely formed to avoid damage to surrounding tissue and organs as well as other adverse complications of surgery.
In cataract surgery and other microsurgical procedures, the lengths of incisions must be very small; and, when forming incisions that are small in length, a knife blade having a known size, or maximum width, as close as possible to the length of incision desired is usually inserted in tissue in a direction normal thereto in the manner of a plunge cut to form an incision in the tissue surface extending lengthwise between the lateral sides of the blade. In other words, an incision having an end-to-end length corresponding to the known width of the blade is formed in the tissue surface when the blade is inserted deep enough to penetrate the tissue surface to the known blade width. Because individual knife blades are conventionally sized to reflect a single, known blade width, an individual blade can precisely form only a single incision of a predetermined width. The lengths of incisions that can be precisely formed utilizing conventional surgical knife blades are limited due to the knife blades being manufactured in a limited number of sizes, or widths. Consequently, in many cases the actual lengths of incisions made with surgical knife blades must be subjectively estimated during incision formation to approximate the optimal incision length, and the actual length of an incision thusly formed is not known absent the use of extraneous measuring devices.
In cataract surgery, the length of an incision made in the sclera or adjacent tissue must be large enough to provide access for lens removal yet no larger than necessary to avoid distortion of the curvature of the eye, or astigmatism, when the incision is closed. In lens removal and replacement surgery of the eye, an incision is made in the eye to be only large enough in length to permit removal of the natural lens due to a blindness causing condition, such as cataract. The optimal length for the incision is very small, i.e. approximately 3 mm, and a surgical knife blade having a known size, close to 3 mm is selected for forming the incision or a thin blade is used with a lateral cutting movement. With a surgeon manipulating the blade via the handle thereon, the tip of the blade is utilized to initially penetrate the sclera, and the blade is inserted while calipers set to 3 mm are held adjacent the incision to compare actual incision length with the calibrated length. If the length of the incision is smaller than desired, the blade is manipulated and incision length measurements are repeated until the proper length incision has been obtained. Once the incision is determined to be accurately formed, the blade is removed, and a surgical instrument is introduced through the incision to remove the natural lens in accordance with a procedure selected for lens removal, such as phacoemulsification. After the natural lens has been removed, a lens implant selected to replace the natural lens is inserted through the incision; and, in most cases, the initial incision must be lengthened to accommodate the implant. Usually, the length of the initial incision must be enlarged to at least 4 mm and, more typically, to approximately 5 mm. A second blade with a known size, as close as possible to the minimum length incision required to accommodate the implant is inserted in the initial incision in a direction normal to the sclera or the incision is enlarged with a smaller blade. As with the initial incision, calipers set to the desired length for the final incision are employed to compare the enlarged incision length with the desired length. Once the desired incision length if formed, the implant is inserted through the incision into the eye. The need for multiple blades having known sizes closely matching desired incision lengths and for measuring instruments to ascertain actual incision length significantly complicates and protracts the surgical procedure while the use of a blade smaller than the desired incision leads to inaccurate incision lengths.
Surgical blades suitable for use in microsurgery are exemplified by U.S. Pat. No. 4,793,218 to Jordan et al. The blades are provided with markings at spaced intervals along a single, tapered lateral edge thereof for alignment with a tissue surface to indicate the depth of the blades in tissue and thusly prevent damage to internal organs from the blades being inserted too far when forming incisions. However, the actual length of an incision produced with the blade when any one of the markings is aligned with the tissue surface is unknown. Additionally, if the blade is inserted in a tissue surface in a direction other than normal thereto, incisions of varying lengths are produced for the associated blade depth depending upon the angle of insertion of the blade.